app: make the backtrace GUI actually work on Win32.
It was previously untested, hence as expected needed fixes. First I add our own exception handler using Win32 API SetUnhandledExceptionFilter(). Second, I reorder things so that ExcHndlInit() is run after this setter, since they will be executed as a FILO and we need backtraces to be generated before our separate GUI runs. Last I run the backtrace GUI as async. No need to keep the main GIMP waiting since the traces have already been generated into a separate file. Also replace gtk_show_uri() by the implementation taken straight from our web-browser plug-in, since apparently gtk_show_uri() doesn't work in Windows (and probably not macOS either since I see we have a separate implementation for this platform as well). I would like to be able to use the PDB but can't because this code needs to be usable both within the main process and into a separate tool process. Ideally, this should just be a utils function which could be included without a problem.
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